Solar flares show their true colors

Solar Flares Show
Their True Colors

New research points to a common mechanism
for spectral behavior in Solar Flares

June
2, 1999: A tool for taking the fingerprints of gamma-ray
bursts from deep space is being used to study the spectra of
flares from the Sun. But unlike humans and gamma-ray bursts,
whose fingerprints are as unique as...well...fingerprints, new
research shows that the detailed spectral behavior of solar flares
falls, for the most part, into just two categories.

Right: An artist's concept depicts a flare evolving
into a Coronal Mass Ejection. Color-color diagrams may help scientists
predict which flares will evolve into potentially dangerous Coronal
Mass Ejections (CMEs), and which ones will fade back into the
solar atmosphere. Links to 930x730-pixel,
90KB JPG. Credit: NASA/Marshall Space Flight Center.

This finding, presented today at the Centennial Meeting of
the American Astronomical Society in Chicago, could yield new
insight into how particles may be accelerated to high energies
in solar flares. It could become especially important over the
next few years as the Sun's activity peaks during solar maximum.

"Right now, we can basically account for the gross properties
of solar flares in our numerical simulations," explained
Dr. Elizabeth Newton, a solar physicist at NASA's Marshall Space
Flight Center. These "gross properties" are like having
a basic understanding that people have two arms, two legs, and
walk upright. It's when you look more closely that people begin
to take on different characteristics, like fingerprints. The
same is apparently true with flares, but only to a point.

Model behavior
"When we 'build a flare' in a computer simulation, we can
reproduce things like the total number of particles being accelerated,
the energies they attain, and the time scales over which these
things occur," remarked Newton. So the next logical step
in the development of their understanding is to probe the details,
and see if their computer models can meet more detailed observations,
such as how the distribution of emitted energy (called a spectrum)
varies with time in a flare.

"We call this variation with time 'spectral evolution,'"
Newton continued. "Is it the same in every flare? Are they
all different? Are there 'classes' of flares? This is what we're
after."

Solar flares are tremendous explosions on the surface and
in the atmosphere of the Sun. In a matter of just a few seconds
they heat material to many millions of degrees and release as
much energy as a billion megatons of TNT.

They occur near sunspots, usually along the dividing line
(neutral line) between areas of oppositely directed magnetic
fields, where the fields have become stressed (sheared). In some
cases, these flares are associated with eruptions of the Sun's
matter into space called "coronal mass ejections."
These events release a million tons of particles traveling at
a 1.6 million km/h (1 million mph) - occasionally directed at
the Earth.

In a solar flare, the released magnetic energy accelerates
particles - electrons and protons - to extremely high energies.
When these particles crash into the solar atmosphere, their kinetic
energy is converted into X-rays and gamma-rays that are detected
by orbiting satellites. The spectral evolution of a flare can
therefore be thought of as a direct fingerprint of the mechanism
that accelerates particles to high energies in the Sun and the
type of target they interact with.

"It's a mark of the throttle for getting these particles
up to high energy in a flare and what material these particles
are interacting with," remarked Newton. "And what we've
found is that there is striking similarity from flare to flare
in how the X-rays these particles produce vary in time and energy."

Flares usually make little difference to the Sun's brightness
in visible light, but stand out well in ultraviolet, X-rays,
and gamma-rays. Their output at higher energies can affect the
Earth's outer atmosphere and damage electronics on satellites.
Scientists have a classification scheme for a flare's intensity
that relates to the total X-ray flux or optical brightness. But
it does not describe how the energies within the overall X-ray
emission are distributed.

As a first step to tell whether a flare's evolving X-ray energy
distribution carries clues about its cause, Newton borrowed a
technique called "color-color diagrams" that was developed
for studying X-ray binary stars. Tim Giblin, a graduate student
working at NASA/Marshall, earlier applied
it to gamma-ray bursts and found that bursts fell into a
half-dozen or more patterns.

Plotting in color
In a color-color diagram, a scientist plots an event's count
ratio, or "color", in one energy band against its count
ratio in another energy band. It's not unlike using your stereo
system's graphic equalizer and taking the ratio of the sound's
loudness in two "tweeter" channels and comparing it
to the ratio of the sound's loudness in two "woofer"
channels. Instead of the amplitude of sound, however, scientists
use the diagram to look at the brightness of X-rays and gamma-rays.

"These diagrams are very useful," commented Newton,
"as they don't rely on any assumptions about what the spectrum
looks like. They're completely model-independent, empirical tools
for examining what's going on in the flare." Previous investigations
into spectral evolution have had to assume a spectral model for
the flare before characterizing the evolution.

Both Newton and Giblin use data from the Burst and Transient
Source Experiment (BATSE) aboard the Compton Gamma-Ray Observatory.
While BATSE was designed to seek the locations of gamma-ray bursts
in deep space, it will detect any gamma-rays or X-rays from any
object in space, including the Sun.

BATSE detects radiation from flares between 20,000 and 1.87
million electron volts (slightly higher than the energy in the
photons in a typical dental X-ray), and divides it into sixteen
energy channels, providing multiple 16-channel snapshots of a
flare each second. Newton took a ratio of the brightness in lower
energy channels and the ratio of brightness in the higher energy
channels, and plotted the two ratios against each other. To help
the human eye track the flare in time from start to finish, a
different visual color is assigned to each point in time
for which one takes the brightness ratios.

Two views of the same event
Newton analyzed 114 flares that were detected by BATSE and also
seen by the Hard X-ray Telescope aboard the Japanese Yohkoh solar
physics satellite, thus providing two independent views of the
same events on the Sun. To this point, however, the analysis
has only focused on the higher sensitivity data provided by BATSE.

Of
36 flares with sufficient data in the highest energy channels,
Newton found that about 80 percent of the color diagrams fall
into a "Lazy V" pattern, and 18 percent fall into an
"Inverse Crescent" pattern. Data on the remaining 2
percent were not sufficient to fall clearly within either category.
This behavior of flares is in marked contrast to their "cosmic
cousins" the gamma-ray bursts, which show a half-dozen or
more distinct patterns in their spectral evolution.

Left:The two graphs at left show the brightness
profiles of two solar flares as detected by BATSE. The first,
on Nov. 10, 1991, was a strong M7.9 flare; the second was a slightly
weaker M4.1 flare on Dec. 4, 1991. At right are their color-color
diagrams showing the "Lazy V" and "Inverted Crescent"
patterns that dominate some 36 flares analyzed with this tool.
Links to 650x551-pixel, 95K JPG.
Credit: NASA/Marshall

The color diagram results don't point directly to an answer
to the mystery of what accelerates material in flares, but it
will help scientists focus their investigations.

"We need to fully characterize the behavior first before
we can explain it," commented Newton. "We hope to find
some relationship between a flare's color-color diagram and the
flare's type. We'd like to predict which ones will cause coronal
mass ejections and which ones will just be a flash in the pan.

"In other words, in order to accurately model what accelerates
particles in solar flares, we have to know observationally what
baseline behavior the model is supposed to meet. We're not seeing
anything that would support a model of random acceleration, as
that would likely result in all the flares' color diagrams being
as unique as fingerprints. There's some sort of local control
involved in how the X-rays are produced, and this local control
seems to be the same in a significant majority of flares."

Scientists are looking forward to the launch of the next instrument
to be applied in this area, the High Energy Solar Spectroscopic
Imager (HESSI) scheduled for July 4, 2000. HESSI's primary mission
is to explore the basic physics of particle acceleration and
explosive energy release in solar flares and will provide better
spectral and temporal resolution, in addition to images of flares.